In the past, pulse phase modulation (PPM) was the method used in infrared transceivers to control actuation of household electronic devices and the like using infrared rays. In pulse phase modulation, data code, in which control data to control the device to be controlled is coded, undergoes PPM modulation into code modulated signals in which differing pulse pause intervals Tr are set based on the values of that bit data. In order to prevent mixing signals with those from other devices and causing erroneous actuation, the 2-bit PPM modulated signal is transmitted by infrared rays to the device to be controlled using a 38 kHz carrier modulated wave as a secondary modulation signal.
This pulse phase modulation method generally used in the past expressed data with a pulse pause interval Tr for each bit. As a result the transmission period TD for sending all of the code became lengthy, and the transmission speed was slow.
As disclosed by Japanese Patent No. 3153084, Applicants previously invented a pulse modulation method that is set by dividing the code into 2-bit data units, and by correlating a differing pulse pause interval Tr to each 2-bit data unit. The transmission period TD of all the modulated code modulation signals is compared to the period TD when all of the bit data of the code has been inverted, and if the transmission period TD has been shortened by inverting, an inversion flag indicating the signal is inverted is added to the code modulation signal produced from the inverted bit data, and this is transmitted.
This pulse modulation method will be explained below using FIGS. 4 and 5. FIG. 4(a) indicates a bit configuration of code to undergo pulse modulation. As indicated in the diagram, 16-bits of ID data and data code are divided into 8 units each comprising 2 bits of consecutive data, and pulse modulation is conducted on each unit. Specifically, a pulse signal with a pulse width t is generated for every unit, which is followed by one of 4 types of pulse pause intervals Tr that are set corresponding to the value of the 2-bit data when each 2-bit data group is taken as a data unit, thereby comprising the 2-bit PPM modulated signal indicated in FIG. 4(b). Further, as indicated in FIG. 4(c), in order to prevent mixing signals with those from other devices and causing erroneous actuation, the 2-bit PPM modulated signal is transmitted by infrared ray to the device to be controlled using a 38 kHz carrier modulated wave as a secondary modulation signal.
As indicated by the normal mode in FIG. 5, in this conventional example, after the pulse signal of pulse width t has been generated, if the 2-bit data is (0,0), the pulse pause interval Tr is set to the same interval as the pulse width t; if (0,1), the pulse pause interval Tr is set to an interval of 2t; if (1,0), the pulse pause interval Tr is set to an interval of 3t; and if (1,1), the pulse pause interval is set to 4t.
The transmission period TD of the code modulated signal modulated in this way varies between 16 t, if all of the 2-bit data is (0,0), and 40 t, if all of the 2-bit data is (1,1). Specifically, the transmission period TD will vary depending on the value of the 16-bit data of the code, and will be shorter if most of the 2-bit data contained is assigned short pulse pause intervals Tr, and conversely, will be longer if most of the 2-bit data contained is assigned long pulse pause intervals Tr.
The transmission period TD generated from 4N-bit data (N is a positive integer) has a reverse threshold value X of 7Nt depending on the value of the bit data. The pulse widths t contained in the pulse intervals Tu of the units of the transmission period TD are equal in every unit comprising 2-bit data, and therefore, if the sum total of pulse pause intervals Tr of the code modulated signal is at least [5N+1] t or more, the transmission period TD can be shortened by inverting the 4-bit data. As indicated by the inversion mode in FIG. 4(d), the values of all the bit data are inverted, and an inverted 2-bit PPM modulated signal code modulated signal is generated using the pulse pause interval Tr (FIG. 4(e).
Next, the fact that the value of the bit data has been inverted and modulated is included in the 2-bit PPM modulated signal as expressed by the flag (0,1), and then transmission becomes possible using a transmission period TD in which the overall code has been shortened.
The code modulated signal (2-bit PPM modulated signal) used in infrared remote control transmitters such as general household electronic products are transmitted to the devices to be controlled using secondary modulated signals that modulate 38 kHz carrier waves. After the device to be controlled has received and photoelectrically converted the infrared light containing the secondary modulated signals, the signals are amplified by an amplification circuit, and as indicated in FIG. 6(b), the amplified output is compared with a specified threshold value Vref, and is demodulated into the code modulated signals indicated in FIG. 6(c).
Depending on the characteristics of the amplification circuit, the rising and falling of the pulses of the demodulated code modulated signals will not be constant because the respective T1 and T2 delays as well as the rise and fall will vary depending on the affects of the light emission element, light receiving element, frequency characteristics of the modulation and demodulation circuit, the transmission route, and the like. For that reason, the rise interval between the pulses of the code modulated signal is taken as the pulse interval Tu of the pulse unit comprising 2-bit data, and the 2-bit data of the demodulated code modulated signals are demodulated from the pulse interval Tu.
Nonetheless, even with the PPM modulation method that demodulates 2-bit data from the pulse interval Tu of the pulse unit, demodulation errors may occur depending on variations of the rise and fall of the pulses based on the transmission environment. For example, the distance between the infrared remote control transmitter and the device to be controlled is short range, the amount of light received will increase and the amplification output voltage will rise. As indicated in FIG. 6(c1), the pulse rise time T3 will be earlier than T1, and conversely, the fall time T4 will be much later than that of T2 because the time of reaching the threshold value Vref or less is delayed.
In this case, the pulse width Pw′ (FIG. 6(c1)) is mainly increased by the delay of the fall time T4, and therefore, if that increased portion exceeds the pulse pause period Tr of the code modulated signal prior to modulation, that pulse fall will overlap with the following pulse rise, thereby causing a demodulation error.
The increased portion of the pulse width Pw′ generally does not exceed the pulse width t of the code modulated signal, which is set to a length of 12 times the cycle tc of the carrier wave in the infrared modulated signal, and therefore, in the pulse modulation method described above, the shortest pulse pause interval Tr of the code modulated signal is set equivalent to the pulse width t, and demodulation is possible even if the fall time T4 is delayed. However, because 4 types of pulse pause intervals Tr proportional to the pulse width t are set corresponding to the 4 combinations of the 2-bit data, a pulse interval Tu 5 times the maximum pulse width t is assigned to the 2-bit data, and the transmission period TD cannot be fully shortened.
Thus, a method was proposed in Japanese Patent No. 3153084 for shortening the transmission period TD of the overall code modulated signal by multiplying the pulse pause intervals by a compression constant k of 1 or less equivalent respectively to the 4 types of pulse pause intervals Tr that are proportional to the pulse width t. However, because the pulse pause interval Tr in the pulse unit in which the shortest pulse pause interval was set became less than the pulse width t of the original signal, not only was there the risk of generating the demodulation error described above, but also, in an environment where the amount of light of the infrared modulation signal received is reduced, there was the problem of mistakenly demodulating to 2-bit data for which a different pulse pause interval Tr was set.
Specifically, if the distance between the infrared remote control transmitter and the device to be controlled is long range, the rise of the amplification output voltage will be delayed because the amount of light received by the device to be controlled has decreased. As indicated in FIG. 6(c2), the pulse rise time T5 will be later than T1, and conversely, after the received light has died off, the fall time T6 will be much earlier than that of T2 because the time of reaching the threshold value Vref or less will come sooner. Under this kind of transmission environment, the pulse rise time T5 is unstable, and a maximum deviation of about ½ of the pulse width t prior to modulation is generated in the pulse interval Tu of the demodulated pulse array. Consequently, if shortening the interval by multiplying the 4 pulse pause intervals Tr, which have been set corresponding to the 2-bit data, by a compression constant k that is 1 or less, there was the risk of producing a demodulation error depending on the transmission environment.